(a) Technical Field
The present disclosure relates to a hill start assist control method and system for vehicles, and more particularly, to a hill start assist control (HAC) method and system for vehicles that prevent malfunction and sensitive operation of an HAC function generated due to misrecognition of a flat road as a hill when the vehicle is accelerated after braking on the flat road.
(b) Background Art
In general, to stop a vehicle on a hill (e.g., an uphill road) and to then restart the vehicle, a driver releases a brake pedal and engages an accelerator pedal and, at this time, when driving force is insufficient, the vehicle moves backwards, i.e., in the downward direction of the hill. Therefore, hill start assist control (hereinafter, referred to as “HAC”) has been proposed, in which, when a vehicle stopped on a hill is started, a brake device provides braking force to respective vehicle wheels to prevent the vehicle from moving backwards.
Such HAC is a function of applying braking force to respective vehicle wheels before driving torque enough to prevent a vehicle from moving backwards is generated from a power unit of the vehicle, and then releasing the braking force, when a driver recognizes a hill (e.g., when the hill is detected) and an accelerator pedal is engaged. The HAC function is operated in a stopped state of the vehicle or in a brake off state of the vehicle in which a brake pedal is released (e.g., pressure is no longer exerted onto the pedal). Additionally, the HAC function is one of the additional functions of an electronic stability control (ESC) system which maintains liquid pressures (e.g., braking pressures) of wheel cylinders allowing an ESC hydraulic modulator to generate braking forces of wheel brakes before driving torque required for climbing a hill is generated, and thus prevents the vehicle from inadvertently moving backwards.
Further, to measure a gradient of a road and to detect whether the road is a flat road or a hill in an HAC system, a longitudinal acceleration sensor (G sensor) is used and a controller of the HAC system is configured to detect and recognize a hill having a designated gradient or more using an acceleration value sensed by the longitudinal acceleration sensor (hereinafter, referred to as “the acceleration sensor”).
Fundamentally, the acceleration sensor outputs 0 as an acceleration value sensed when a vehicle drives at a constant velocity on a flat road, outputs a positive (+) or negative (−) acceleration value when the vehicle accelerates or decelerates on a flat road, and outputs a positive (+) or negative (−) acceleration value when the vehicle drives at a constant velocity or stops on an uphill road or a downhill road, as exemplarily shown in FIG. 1 of the related art. The gradient (θ) of a road is an input in the operation of the HAC system. Whether the road is a flat road or a hill is determined by calculating the gradient of the road from a value sensed by the acceleration sensor, a necessary braking pressure is determined based on the gradient value when the gradient is greater than a set value, and liquid pressures (braking forces) of wheel cylinders are adjusted through a valve of the ESC modulator to track the braking pressure as a target value.
Additionally, the HAC malfunctions on a flat road more frequently than on a hill since when a brake pedal is engaged to suddenly brake a vehicle, nose dive is generated and, when the brake pedal is disengaged or the vehicle is accelerated after sudden braking, nose up is generated, and thus, the acceleration sensor outputs the same value as the value, which is output on a hill. In other words, when a vehicle is suddenly decelerated and then driven on a flat road, behavior of the vehicle in the pitch direction is generated, as exemplarily shown in FIGS. 2A and 2B of the related art, the flat road may be misrecognized as having a gradient and the HAC function may be unnecessarily operated.
In particular, even though the vehicle may be traveling on the flat road, the value sensed by the acceleration sensor may cause a determination that the vehicle is traveling on a hill and, thus, a valve actuator of the ESC hydraulic modulator may be operated to adjust liquid pressures of respective wheel brakes. Consequently, the hydraulic modulator may be operated on the flat road and, thus, the vehicle may be decelerated, noise caused by operation of the valve of the hydraulic modulator and noise caused by friction between a disc and a pad may be generated and the vehicle may be unstable.